Canterbury Bells

Bellflowers or harebells are one of the classic plants associated with the English country garden. For today's post, I'll be covering the family of plants that bellflowers belong to.

Fairy's thimble Campanula cochleariifolia, copyright Jerzy Opioła.

The Campanulaceae are a family of over 2300 plant species found almost worldwide (Crowl et al. 2016). The family is, however, divided between five subfamilies that some authors would treat as separate families, in which case 'Campanulaceae' would be restricted to the 600 or so species of the subfamily Campanuloideae. It is this subfamily that includes the bellflowers. The vernacular name, of course, refers to the shape of the flowers produced by these plants, as indeed does the botanical name: Campanula translates as 'little bell'. These flowers are radiately symmetrical with all petals more or less the same size and shape and evenly arranged in a circle. Other subfamilies of the Campanulaceae in the broad sense, the largest of which is the lobelias of the Lobelioideae, produce more bilaterally symmetrical flowers with petals differing in size and/or with some petals closer together than others. Fruits are most commonly a capsule, with the seeds dispersed by wind, but some lobelioids produce fleshy fruits that attract birds. The lobelioids are most diverse in the southern continents, and it is thought that this may have been the original home of the family as a whole when it arose sometime close to the end of the Cretaceous, possibly in Africa. At some time in the early Cenozoic, however, the campanuloids arrived in and underwent a significant radiation in the Palaearctic. This dispersal may be related to the different flower morphology of the campanuloids, as they adapted from the bird, bat and butterfly pollinators of the tropics to the bee and fly pollinators of more temperate habitats.

Glandular threadplant Nemacladus glanduliferus var. orientalis, copyright Stan Shebs.

The genetics of Campanulaceae, specifically of their chloroplasts, should also not go unnoticed. The structure of the chloroplast genome in plants is usually very stable, with few changes in gene arrangement and order. However, at various points in the history of Campunulaceae, large chunks of foreign DNA have been inserted in the original plastid chromosome, with a number of these insertions also associated with inversions in the direction of adjoining sections of the original genes (Knox 2014). This kind of insertion is unique among flowering plants: changes in the gene content of plastids more usually involve genes being transferred out of the plastid. The source of this extra DNA is uncertain: it may have come from the plant's own nucleus, or it may have come from an as-yet-unknown endosymbiont. Also unknown is the functional significance of these rearrangements, if any. Some insertions have clearly resulted in pseudogenes, with their sequences rapidly breaking down through subsequent genetic drift. But others have preserved the structure of functional genes, suggesting continued selection for their retention.

Cyanea duvalliorum, an arborescent Hawaiian lobeliad, copyright Forest & Kim Starr.

The majority of Campanulaceae are small perennial herbs. Two genera of distinctive enough to be assigned to their own subfamilies include annual herbs: the threadplants Nemacladus of southwestern North America, and the little-known Chilean Atacama desert endemic Cyphocarpus. Some members of the Lobelioideae are woody subshrubs, and at some point one of these woody lobelioids managed to make its way to the Hawaiian archipelago where it gave rise to one of the world's most remarkable insular radiations, and the single largest such radiation in plants. Over 120 species of lobeliads are known from the Hawaiian islands, varying from single-stemmed succulents to straggling vines to trees over 18 metres in height. There are inhabitants of lowland forests, of upland bogs, and of rocky cliffs. There are species producing fruit as dry capsules; others produce fleshy berries. So varied are the Hawaiian lobeliads that previous authors have inferred their origin from multiple seperate colonisations, but a study by Givnish et al. (2009) supported a single origin from a single colonist arriving about thirteen million years ago. This would have been before any of the current major Hawaiian islands existed (the oldest, Kaua'i, is a little less than five million years old); the implication is that the ancestor of the Hawaiian lobeliad arrived on a pre-existing island, perhaps corresponding to the modern Gardner Pinnacles or French Frigate Shoals. As the lobeliads diversified, they continued to disperse onto new islands as they arrived, while their original homeland eroded away.

Sadly, a depressing percentage of the species forming this incredible radiation are now threatened with extinction, the victims of pressures such as loss of habitat, the decline of their pollinators and dispersers, or grazing by introduced mammals. The cliff-dwelling pua 'ala Brighamia rockii of Moloka'i is now restricted to five locations with an estimated total wild population of less than 200 individuals. A related species on Kaua'i, the olulu Brighamia insignis, may be extinct in the wild, having last been recorded in the form of a single individual in 2014 (it still survives in cultivation). As we earlier saw with the Hawaiian honeycreepers, there is barely a single section of the Hawaiian biota not marked by tragedy.

REFERENCES

Crowl, A. A., N. W. Miles, C. J. Visger, K. Hansen, T. Ayers, R. Haberle & N. Cellinese. 2016. A global perspective on Campanulaceae: biogeographic, genomic, and floral evolution. American Journal of Botany 103 (2): 233–245.

Givnish, T. J., K. C. Millam, A. R. Mast, T. B. Paterson, T. J. Theim, A. L. Hipp, J. M. Henss, J. F. Smith, K. R. Wood & K. J. Sytsma. 2009. Origin, adaptive radiation and diversification of the Hawaiian lobeliads (Asterales: Campanulaceae). Proceedings of the Royal Society of London Series B—Biological Sciences 276: 407–416.

Knox, E. B. 2014. The dynamic history of plastid genomes in the Campanulaceae sensu lato is unique among angiosperms. Proceedings of the National Academy of Sciences of the USA 111 (30): 11097–11102.

Alvania

It's a general rule with organisms that species diversity increases as size decreases (at least down to about the millimetre range, below which things get a bit more complicated). That's certainly the case with molluscs, whose range clearly favours the tiny.

Alvania cimex, copyright Alboran Shells.

Alvania is a cosmopolitan genus of marine gastropods, found in most parts of the world except the Antarctic and sub-Antarctic (Ponder 1984). The average Alvania species is less than five millimetres in total length, and other members of the family they belong to, the Rissoidae, are similarly wee. The shell of Alvania species varies from elongate-conical to more squatly conical in shape, and generally has a sculpture of both axial and spiral ridges. In some species the axial and spiral ribs are both similarly prominent; in others, the spiral ridges are more strongly developed.

Rissoids may be found crawling on seaweed or sheltered amongst stones or other rubble. Alvania species seem to be more likely to be found in the latter habitat than the former. Alvania have a smaller mucous gland on the rear of the foot than species of Rissoa, a related genus that is more likely to be found on the weeds. The mucus produced by this gland assists rissoids in clinging to their substrate or the surface film, and its reduction in Alvania is presumably connected to their preference for the low life. Rissoids are grazers on microalgae or deposit feeders; those species found on seaweeds will feed on diatoms and the like growing over the seaweed rather than on the seaweed itself. Among European species, A. punctura is known to selectively pick out diatoms and dinoflagellates from among detritus when feeding whereas A. jeffreysi may be less discriminating in what it swallows.

Alvania subcalathus, copyright H. Zell.

The greater number of Alvania species are planktotrophic as larvae, and as described in some of my previous posts on turrids, their shells have protoconches to match. Nevertheless, the genus also includes some direct-developing species with fewer protoconch spirals. The Mediterranean species A. cimex and A. mammillata are almost indistinguishable when mature except by features of the shell apex, which is broader with fewer spirals to the protoconch in the latter (Verduin 1986). If A. mammillata is a direct developer while A. cimex has a planktotrophic larva, it would tally up with the situation elsewhere seen among turrids.

REFERENCES

Ponder, W. F. 1984. A review of the genera of the Rissoidae (Mollusca: Mesogastropoda: Rissoacea). Records of the Australian Museum Supplement 4: 1–221.

Verduin, A. 1986. Alvania cimex (L.) s.l. (Gastropoda, Prosobranchia), an aggregate species. Basteria 50: 25–32.

Niphargus

I've commented before on the unexpectedly high diversity of animal species that can be found living in groundwater. Because dispersal through this habitat is, unsurprisingly, often difficult and bodies of groundwater are often isolated from each other, many groundwater-adapted species can have almost ludicrously small ranges. Hence, for instance, the diversity of the amphipod genus Niphargus.

Niphargus hadzii, copyright B. Sket.

The genus Niphargus is found across Europe, mostly south of what was the lower edge of the northern ice sheet during the Pleistocene, though it is replaced by a closely related genus in the Iberian Peninsula. A few species are also found in south-west Asia. They are usually eyeless and colourless (the genus name means 'snowy white'). Though clearly adapted for subterranean environments, they may also be found in associated surface habitats such as springs or the upper sections of streams (Fišer et al. 2015). Species of Niphargus vary considerably in size: the smallest interstitial species may be only two or three millimetres in length whereas some cave-dwellers reach forty millimetres (Karaman & Ruffo 1986). They may feed on organic particles filtered from the water, or they may predate on smaller animals.

Over 300 species of Niphargus are currently recognised, making it one of the largest genera of freshwater amphipods. Even so, this number is likely to be a significant underestimate of the genus' true diversity. A number of studies on Niphargus have identified evidence of previously cryptic species. A study by Fišer et al. (2015) of two species from the Istrian Peninsula in the north-west Balkans found genetic evidence for the existence of two strongly divergent populations within each, with the two populations of 'N. krameri' being morphologically as well as genetically distinct. Flot et al. (2010) found evidence of four distinct lineages within 'N. ictus' of Italy's Frasassi caves, representing at least three independent colonisations of the cave system from external sources. At least two of these lineages are unique among amphipods in living in a symbiotic association with sulphide-oxidising bacteria of the genus Thiothrix, allowing them to survive in Frasassi's sulphide-rich waters.

Niphargus aquilex from the River Till, copyright Lee Knight.

With such a large genus, attempts have naturally been made to divide it into more manageable units. About a dozen species groups have been recognised on the basis of morphology, albeit some poorly defined. However, a molecular phylogenetic study of the genus by Fišer et al. (2008) identified none of these groups as monophyletic. Instead, they found a higher correlation of phylogeny with geography than morphology. The picture suggested is one of poor dispersers, re-evolving similar forms on multiple occasions as they diverge to exploit their secluded habitats as best they can.

REFERENCES

Fišer, C., B. Sket & P. Trontelj. 2008. A phylogenetic perspective on 160 years of troubled taxonomy of Niphargus (Crustacea: Amphipoda). Zoologica Scripta 37 (6): 665–680.

Fišer, Ž., F. Altermatt, V. Zakšek, T. Knapič & C. Fišer. 2015. Morphologically cryptic amphipod species are "ecological clones" at regional but not at local scale: a case study of four Niphargus species. PLoS One 10 (7): e0134384.

Flot, J.-F., G. Wörheide & S. Dattagupta. 2010. Unsuspected diversity of Niphargus amphipods in the chemoautotrophic cave ecosystem of Frasassi, central Italy. BMC Evolutionary Biology 10: 171.

Karaman, G. S., & S. Ruffo. 1986. Amphipoda: Niphargus-group (Niphargidae sensu Bousfield, 1982). In: Botosaneanu, L. (ed.) Stygofauna Mundi: A Faunistic, Distributional, and Ecological Synthesis of the World Fauna inhabiting Subterranean Waters (including the Marine Interstitial) pp. 514–534. E. J. Brill/Dr W. Backhuys: Leiden.